Supplication for if you are stricken by doubt in your faith

أَعُوْذُ بِاللّٰہِ

I seek refuge in Allah.

[Sahih Bukhari – 3176]

اٰمَنْـتُ بِاللهِ وَرُسُـلِهٖ

I believe in Allah and His Messenger.

[Sunan Abu Dawud – 2357]

هُوَ الْأَوَّلُ، وَالْاٰخِـرُ، وَالظَّـاهِـرُ، وَالْبَاطِـنُ، وَهُوَ بِكُلِّ شَيْءٍ عَلِـيْمٌ

He is the First and the Last, the Most High and the Most Near. And He is the Knower of all things.

[Abu Dawud 4/329]